As the Librarian of the Chromatic Dragons' library, Fati had every part of this island under her control.
But Marvin was told by the Dragon Soul that there was one place that wasn't under Fati's control!
That was the Sea of Nothingness outside the island.
The Sea of Nothingness looked like a sea, but it was in essence a space separating the library from the external plane.
This space was similar to the Shadow Plane.
It acted as a cover and a defense, preventing outside lifeforms from entering the library. Marvin could effectively avoid Fati's attention there.
The only complication was that there were potential unknown existences in the Sea of Nothingness.
Moreover, the Dragon Library was equivalent to a Demi-Plane, and the borders of planes always had various kinds of things, some of which could be dangerous.
But Fati had the advantage right now, so Marvin could only take a risk.
Sure enough, he was successful.